Printed circuit boards are the common building block of virtually all modem day electronic equipment. Typical circuit boards are comprised of at least one layer of insulative material, such as FR-4 or glass epoxy, which has one or both of its major surfaces clad with metallization, e.g., copper or the like. The metallization on each surface is patterned, usually by well-known photolithographic techniques, to establish component-mounting areas (pads or metallized throughholes) which are selectively linked to each other by metallized connecting paths.
To allow mounting of more components on a circuit board of given area, the size and spacing between the metallized connecting paths is continually being reduced. In the past, the standard minimum spacing between the metallized connecting paths was on the order of 8 mils. Today, this is getting smaller.
The continued reduction in the spacing between the metallized connecting paths has required a corresponding reduction in allowable manufacturing tolerances. To achieve reduced manufacturing tolerances, more accurate techniques for inspecting the metallized connecting paths on the circuit board surface(s) must be employed if opens, shorts and design rule violations are to be detected. A common approach for accomplishing such automated inspection is to span the circuit board with a beam or line of light and measure the intensity of the light reflected from the board surface. Ideally, the metallized connecting paths and component-mounting areas ("features") on the circuit board should exhibit a much higher reflectivity than the circuit board itself, causing the light reflected from the features to have a greater intensity. However, the metallized features are sometimes tarnished and/or corroded so their reflectivity may be significantly decreased. As a consequence, the actual difference in the intensity (i.e., the contrast) of the light reflected from a metallized feature and from a non-metallized area on the circuit board may not be very great. When the contrast is small, the accuracy of the inspection is reduced.
In my now-allowed U.S. patent application Ser. No. 440,948, filed on Nov. 24, 1989, and assigned to the present assignee, a method and apparatus is disclosed for obtaining a three-dimensional image of the surface of a substrate, such as a printed circuit board. The three-dimensional imaging technique disclosed in this application is practiced by separately spanning the substrate with first and second lines of light. Each of the first and second lines of light is directed at the substrate at about the same angle to strike a successive one of a first and second plurality of strips of area, respectively, running across the substrate along a first axis parallel to the plane thereof.
The first and second lines of light each have known intensity profiles which, when normalized, will spatially intersect. In other words, each line of light has a known intensity variation along a second axis perpendicular to the first axis and perpendicular to the plane of the substrate. As each line of light is spanned across the substrate, there will be a line along the substrate at which the normalized intensity profile is the same for each line of light as for the other. As each line of light is spanned across the substrate, a linescan camera also spans the substrate to sense the intensity of the light reflected from a successive one of a set of third strips of area, each spaced close to the line of the intersection of the normalized intensity profiles. The ratio of the sensed intensities yields the height of the features in each successively imaged third strip. In this way, a three-dimensional image of the substrate surface can be obtained.
My three-dimensional imaging technique, as described above, can be employed to measure the height of metallized features. The disadvantage of this approach is that the height of the metallized features tends to be very small. Further, the received intensity profiles from the translucent circuit board surface are significantly different from the profiles attributed to the metallization on the board surface. This is because the light which strikes the non-metallized portions of the translucent board surface will spread within the surface itself, causing a change in the received profile. In general, the height-measurement technique described in my allowed application, Ser. No. 440,948, is not applicable for surfaces whose intensity profiles are different.
Thus, there is a need for a technique for inspecting a substrate, such as a printed circuit board, to detect the metallized areas thereon, which technique is robust and less susceptible to tarnishing and corrosion of the areas.